The invention is based on a fuel injection device for a Diesel engine. In a known fuel injection apparatus of this kind (German Offenlegungsschrift No. 30 02 851), for the sole instance where, on the one hand, a primary fuel that does not ignite readily and is supplied by a high-pressure injection pump and, on the other, an igniting fuel supplied by a separate pump are to be delivered via a hydraulic auxiliary pump to separate injection nozzles for the primary and the igniting fuels in a Diesel engine, it is provided that a pre-injection piston in the auxiliary pump is driven at least indirectly at the supply pressure of the high-pressure injection pump and thereby positively displaces a pre-injection quantity from a work chamber disposed upstream of the work chamber into a separate nozzle for the pre-injection. If the piston in the pressure distributor is embodied as a stepped piston, then the supply pressure of the high-pressure injection pump acts upon the driving piston part, that is, the part having the larger diameter, of the stepped piston, which with corresponding translation mechanically drives the pre-injection piston. Even before the pressure chamber of the driving piston, a branching pressure line leads to the primary injection nozzle. A distribution that is this simple can lead to difficulties in attempting to control the time sequence between pre-injection and main injection accurately, because on the one hand the dead spaces that exist in the connecting lines lead to unavoidable deviations, occurring especially in accordance with load and rpm, from the prespecified control times for the pre-injection and the main injection. At high rpm, for instance, the onset of the pre-injection and the onset of the main injection can take place quite close to one another and in effect combine into a single injection. Means for varying or prespecifying the instant of injection for the pre-injection quantity in an intentional manner are not provided in a timed relationship with the main injection.
It is generally known that undesirable operating noises in Diesel engines are the result of the greater heat developed at the onset of combustion, so efforts have long been made to initiate the combustion by means of a limitable, small pre-injection quantity that can be positioned desirably in terms of time with respect to the main injection, and thereby to limit the combustion speed, or in a general sense to control it. The solution that presents itself in this respect, that is, to dispose two complete, separate injection systems operating parallel to one another is expensive and not recommended, because not only are two pumps, two lines and two nozzles required, but also the necessary means for synchronizing the two systems.
It is also known to attain pre-injection effects by suitable dimensioning of a normal injection system; in that case a specific relationship in terms of size and function must be adhered to for the pre-stroke, diameters of the lines, nozzle ports and nozzle springs, but that leads to a disadvantageous dependency on load and rpm and on the varying dynamic influences during engine operation.
A further known provision is to provide injection pumps with additional control devices, as well as an intermediate reservoir, by means of which the supply speed can be reduced by throttling to the vicinity of zero. In the course of this, an initial stage in the pressure wave traveling to the nozzle can develop, and hence a sort of pre-injection is attainable, at specific rpm and load levels.
Even if the metering and timing of the pre-injection is performed by two systems having two injection pumps, the camshafts of which are coupled together, there are still problems in attaining correct phase orientation between the pre-injection and the main injection as a result of the dependency on rpm and load resulting from the dynamic influence of the two lines.
Still another apparatus is known (German Pat. No. 1 252 001) for attaining a pre-injection and a main injection, in which a separate small piston for the pre-injection is disposed axially offset with respect to a load piston for the main injection inside a fuel injection valve. In this apparatus a separate supply means for low pressure can be dispensed with, and the pre-injection quantity is obtained from the fuel supplied for the main injection, which does not preclude a disadvantageous effect on the standing pressure in the pressure line and thus on the accuracy of quantity control.
Finally, it is also known (German Offenlegungsschrift No. 28 34 633), for controlling the pre-injection in internal combustion engines, to provide a one-piece control slide that is displaceable counter to the force of a spring and that with a pronounced intermediate relief into a reservoir, via control edges, establishes the desired relationships between the pre-injection and the main injection. Here again, the pre-injection is diverted from the injection pump that also furnishes the main injection quantity, so that the accuracy of quantity control for the main injection quantity is disadvantageously affected.